In recent years, various phosphors have been used in plasma display panels (hereinafter referred to as PDPs). Among such phosphors, alkaline earth metal aluminate phosphors containing bivalent europium as an activator, for example (Ba,Sr)MgAl10O17:Eu2+, are used as blue phosphors.
When exposed to elevated temperatures or to vacuum ultraviolet rays, ultraviolet rays or the like for phosphor excitation, such alkaline earth metal aluminate phosphors containing bivalent europium as an activator deteriorate and decrease in luminance. The mechanism so far proposed as the cause therefor consists in the oxidation of bivalent Eu, which forms luminescence centers in the blue phosphors, in particular, to the trivalent form due to the oxidative effect of heating on the phosphor surface, resulting in the loss of bivalent blue luminescence centers and the decrease in luminance.
The phosphors may be subjected to heating at elevated temperatures in the step of molding for use in certain fields of application. In the process of manufacturing PDPs, for instance, partitions called ribs are formed on the back face glass sheet, and the respective fluorescence substances each in the form of a paste prepared by using a binder and a solvent are applied to respective partition-surrounded areas without color mixing. The glass sheet is then heated at 400 to 500° C. for binder elimination in the manner of firing and for fusion bonding to a front face glass sheet. It is suggested that, in such a firing step, the firing is carried out in a high humidity environment as a result of not only the oxidative action but also the evaporation of the moisture originally contained in dielectrics, electrodes and other materials than the phosphors, possibly affecting the fluorescent materials. Therefore, as far as blue phosphors are concerned, it is a great problem to inhibit luminance decreases as well as emission shifting in the heating/firing step.
For deterioration prevention, some attempts have already been made to prevent the deterioration by chemical treatment of the phosphor surface (e.g. Japanese Kokai Publication H10-195428, Japanese Kokai Publication H10-298548, Japanese Kokai Publication H10-204429). However, these methods consist in coating the surface with boric acid or a compound of such an element as antimony or silica and, therefore, it is difficult to entirely prevent the oxidative deterioration and, in addition, a problem arises, namely the luminance decreases due to the covering of the phosphor surface with some other substance.
Another problem is that a phosphor, once colored and reduced in powder whiteness, absorbs the fluorescence emitted, resulting in reduced function. Therefore, it is desirable that the phosphor has high powder whiteness. As means for preventing the luminance of a phosphor from decreasing, there has been disclosed a method of producing phosphors which comprises firing in a reducing atmosphere, followed by firing in an oxidizing atmosphere (e.g. Japanese Kokai Publication 2002-348570). Such method, however, is a method of producing alkaline earth metal silicoaluminate phosphors but is not intended for increasing the powder whiteness of a phosphor.